Photo and Electrochemical Characteristics Dependent on the Phase Ratio of Nanocolumnar Structured TiO2 Films by RF Magnetron Sputtering Technique

• Published in: Chemistry of materials 2009-07, Vol.21 (13), p.2777-2788
• Main Authors: Kang, Soon Hyung, Lim, Ju-Wan, Kim, Hyun Sik, Kim, Jae-Yup, Chung, Young-Hoon, Sung, Yung-Eun
• Format: Article
• Language: English
• Subjects: Electrochemistry; Nanomaterials (Nanops, Nanotubes, etc.); Optical Materials (including Non-Linear Optic, Photonic, and Optoelectronic Materials)
• ISSN: 0897-4756; 1520-5002
• DOI: 10.1021/cm900378c

Nanostructured titanium dioxide (TiO2) prepared by RF magnetron sputtering was grown at substrate temperatures (T sub) ranging from RT (25 °C) to 450 °C. At lower temperatures, the TiO2 film deposited at a high sputtering pressure (12 mTorr) was amorphous. This was due to adatoms being unable to migrate effectively on the substrate. However, an increase in T sub provides additional thermal energy and promotes the formation of the rutile at high temperature. The postannealing process provides additional driving force for nucleating the amorphous phase to anatase or rutile, followed by the growth of the rutile grain size. A 2.2 μm thick TiO2 film sputtered at 350 °C had a columnar structure and showed the best performance as a photoanode layer for a dye-sensitized solar cell (DSSC), with a V oc of 0.65 V, a J sc of 7.93 mA/cm2, a fill factor of 0.533, and efficiency of 2.74%. In addition, the durable electrochromic performance was also confirmed using 400 nm thick TiO2 films sputtered at 350 °C. The improved properties were attributed to the formation of a mixed crystalline phase with anatase and rutile, a porous structure, and the narrowing of the optical band gap because of the formation of rutile, trap sites, and surface states. Furthermore, the calculated porosity of the TiO2 film sputtered at 350 °C also showed a high value (∼60%), which contributes to the increase in surface area and is in close contact to the electrolyte.